I have a world made of many cubes (like in Minecraft), they have only color (not texture). I am rendering them using OpenGL 3.3 core profile (GLFW, GLAD, GLM). I am already have done some optimizations:

• using one VBO for all cubes
• not rendering internal faces
• not rendering faces that can't be visible from camera
• rendering faces using GL_TRIANGLES and indexing

But it's still slower than Minecraft with Optifine, but Minecraft is in Java and blocks have textures (there are entities etc. too)! How can I optimize my program further?
Edit:
I think I might do some work with threading, but I don't know what should different threads do.